The initial stages of W(110) oxidation have been characterized by surface-sensitive W(4f) core-level photoemission, with the surface-sensitivity provided by the use of tunable synchrotron radiation for excitation. Specific W(4f) Photoemission features are assigned to W-atoms bonded to oxygen at coverages below and above 1 2 monolayer (ML). The higher coverage oxygen-induced peaks are at a significantly higher binding energy, which is consistent with an increase in the coordination of W to O and, correspondingly, an increase in the extent of charge transfer between W and O. These results are also consistent with previous temperature-programmed desorption (TPD) studies. Notably, the TPD experiments have demonstrated that only atomic oxygen is desorbed for oxygen coverages (θ0) below 1 2 ML, while suboxides of tungsten (WOx,x=1-3) are observed as desorption products for θ0> 1 2 ML. Changes we observe in the valence-band spectra may also be reconciled within this model. © 1994.
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